Again, this is a controversial topic. One study claimed to demonstrate that MCS does not exist and is really a manifestation of mental illness. However, the study was found to be flawed. 

MCS -- also called Environmental Illness (EI) and "20th century disease" -- is a condition that causes sensitivity to chemicals (by inhalation, ingestion, or contact) to a point where the afflicted individual suffers adverse health effects from exposure to even extremely low levels of a variety of chemicals.

The causes vary with the disease. Examples include Legionnaire's disease if caught from a building's cooling tower; carbon monoxide poisoning from a malfunctioning water heater; etc. In addition to any appropriate treatment for the disease, elimination of the source of the problem is essential and may actually be the only thing required to eliminate the disease.

SBS is the condition of a building in which more than 20% of the occupants are suffering from adverse health effects, but with no clinically diagnosable disease present.

BRI is the general term for a medically diagnosable illness which is caused by, or related to, occupancy of a particular building.  Legionnaire's disease and carbon monoxide poisoning are two examples.

The causes of SBS are still uncertain. Amongst the causes postulated, and for which some evidence exists, include man-made mineral fibers (MMMF), macromolecular organic chemicals (large size molecules produced by living things, such as protein molecules), volatile organic compounds (VOC's), and many others.